1. Field of Invention
The present invention relates to a method to form a functional porous layer, a functional material being supported such that the content of the functional material varies in the depth direction of the porous layer, to a method to manufacture a fuel cell applying the above method to form a reaction layer, and to an electronic device and an automobile having the fuel cell manufactured by the method as a power supply.
2. Description of Related Art
A fuel cell including an electrolyte membrane, an electrode (anode) disposed adjacent to one face of the electrolyte membrane, and an electrode (cathode) disposed adjacent to the other face of the electrolyte membrane is well known in the related art. For example, in a solid polymer electrolyte fuel cell, which includes an electrolyte membrane composed of a solid polymer electrolyte membrane, the following reactions are performed. On the anode side of the fuel cell, hydrogen gas splits into hydrogen ions and electrons, the electrons are conducted to the cathode side, and the hydrogen ions are moved in the electrolyte membrane to the cathode side. On the cathode side of the fuel cell, oxygen gas, the hydrogen ions, and the electrons are allowed to react to form water molecules.
In this solid electrolyte fuel cell, each of the electrodes generally includes a reaction layer composed of metal microparticles, which are a reaction catalyst of reactive gases, a gas diffusion layer composed of carbon microparticles disposed adjacent to the reaction layer toward a substrate, and a current-collecting layer composed of a conductive material disposed between the gas diffusion layer and the substrate. On one substrate (first substrate), hydrogen gas passes through gaps between the carbon microparticles of the gas diffusion layer and is uniformly diffused. The hydrogen gas is then allowed to react at the reaction layer to form electrons and hydrogen ions. The electrons are collected in the current-collecting layer and are conducted to a current-collecting layer on the other substrate (second substrate). The hydrogen ions are moved to a reaction layer on the second substrate through a solid polymer electrolyte membrane, and then allowed to react with electrons that are conducted from the current-collecting layer, and oxygen gas to form water molecules.
According to this fuel cell, for example, the following methods to form the reaction layer are known in the related art. (a) A paste to form a catalyst layer of an electrode prepared by mixing a catalyst supported on carbon, a solution of polymer electrolyte and an organic solvent is applied on a substrate (polytetrafluoroethylene sheet) and the substrate is dried. The dried paste on the substrate is bonded by pressing on an electrolyte membrane with heating. The substrate is then stripped off, thereby transferring the catalyst layer (reaction layer) to the electrolyte membrane Japanese Unexamined Patent Application Publication No. 08-88008. (b) A solution of electrolyte containing a solid catalyst supported on carbon particles is sprayed on a carbon layer, which is used as an electrode and the solvent in the solution is volatilized, thereby forming the reaction layer Japanese Unexamined Patent Application Publication No. 2002-298860.
However, these methods include many steps and are complicated. Furthermore, applying the catalyst uniformly and applying a predetermined amount of the catalyst to a predetermined position accurately are difficult to achieve. Unfortunately, the above methods decrease a characteristic, i.e. power density of the fuel cell and increase the production cost because of the increasing use of an expensive catalyst, such as platinum.